Fuel Throttles, 4 Hole vs Downdraft

[Liberator*21]

Which is more efficient: 2.5L downdraft or a 4 Hole EFI throttle ? And if its the Downdraft, does it have the same bolt pattern as an 90's 2.5 EFI motor ?

Thanks

[Dave Strong]

Downdraft is an OPTI thing, 4 hole a Laser thing. Never seen a Opti ft on a Laser motor before, adding SVS style intake to the 4 hole will give more power.

Dave

[25two.stroke]

What do you mean by more efficient? And what motor are you talking or is it just a general theory question?

Are you talking about the late model downdraft on some efi motors like the 225x and the later fishin 200 hp efi (CDM ignition) compared to the 4 butterfly lazer injection on the earlier efi? If thats what you are talkin about I am not the one to answer it, but I know the gasket is different so I don't think they bolt straight up. Thats not to say you couldn't drall and tap some holes, or weld closed some things, to make it work.

[ChrisCarsonMarine]

down draft flows LOTS MORE. different bolt patterns. does a 2.5 need the more flow though??

[Liberator*21]

Downdraft is an OPTI thing, 4 hole a Laser thing. Never seen a Opti ft on a Laser motor before, adding SVS style intake to the 4 hole will give more power.



Dave

​Thanks Dave

What do you mean by more efficient? And what motor are you talking or is it just a general theory question?

Are you talking about the late model downdraft on some efi motors like the 225x and the later fishin 200 hp efi (CDM ignition) compared to the 4 butterfly lazer injection on the earlier efi? If thats what you are talkin about I am not the one to answer it, but I know the gasket is different so I don't think they bolt straight up. Thats not to say you couldn't drall and tap some holes, or weld closed some things, to make it work.

Thank you Two Stroke

down draft flows LOTS MORE. different bolt patterns. does a 2.5 need the more flow though??

Thank you Chris.
That's pretty much my question, is there any benefit in power to be realized. Reason for asking is, I can get a complete downdraft (225x) for free and I do have the means to adapt pretty much anything to anything, but is it worth it ? Maybe it's over kill ? I'm still new to 2.5's coming in from the 300x world and still learning.

[ChrisCarsonMarine]

Gary,you pose a good question,I think I realized the answer a few years back...coming from carburetors as anyone over 50 did,you understand the one cylinder to one venturi motor design.That one lonely carb throat has to satisfy the needs of its perspective cylinder...only.
Our v6 60 degree motors begin drawing air at #1,then 30 degrees later #2 begins drawing,the two draw together for 60 degrees,then #1drops out and#2 draws the last 30 degrees...then #1 and #2 cylinders are done drawing air and #3 and#4 follow.This 60 degree overlap between #1 and #2 cylinders has the venturi at full flow for both cylinders,but only during the 60 degree overlap,and by cross drilling the intake either(both) cylinders can draw from both carbs at all times,but importantly,during the pre and post 60 degree overlap period when #1 and later #2 benefit from one cylinder drawing from 2 carbs.It gets better...
Now comes the Laser fuel injection airbox,with 4 butterflys.For our airflow purposes,think of them as 4 carb venturies feeding the above 2 cylinders at once,which they are!Lots of air,way more than 1 venturi to one cylinder could provide,unless it was a huge venturi.
The point is,once all cylinders are pulling from a single airbox,the intake on that airbox only needs to be large enough to feed 2 cylinders at any given time.
As to the Downdraft box...in my opinion the design is certainly better,from an airflow potential and production complexity point of view...and I would guess Mercury went with it over the Laser type due to production costs,it is certainly simpler.
Not having done any dyno comparisons I can't say,but my guess is the downdraft probably is'nt a performance enhancer on stock or even moderately modded motors,more flow potential is only potential if it's not needed.The stock downdraft 200's don't seem any better than their laser counterparts in my experiance...we've run both,Chris

[25two.stroke]

Man I'd love to get my hands on a 225x....I would say its worth it to swap it over depending on what you are bolting it on. It seems, Like Chris said, that they wend to it for production cost reasons...making all their motors run the same intake and throttle linkage. What are you planning on doing inside the 2.5 200? You leaving it stock or making it a beast? ^ Chris said it best, but even if its not a performance gain I think it would be cool to do.

[Chaz]

The 4 holes in the Laser add up to more blade area than the single butterfly of a down draft intake. It's downside is that the air has to come in from the back side , make a U-turn .. find it's way past wires , a fuel rail and centrally mounted injectors. those kits that add air from the front tend to confuse the plenum area with colliding columns and still has to deal with the internal mess inside.

The downdraft plenum is a big "still airbox" as it holds holds the same in not more cubic inches of displacement than the engine it provides air for as well as moving the injectors to the outside making for a straight shot from box to reed cage.
The throttle body only needs to be able to replace the volume of air drawn by a single intake signal. I don't think you could rev the motor high enough to run the plenum dry let alone find the limits of the throttle body.

Slides run ratty down low , not all of the opening's seal the same , so some holes run lean while others cut of the air and start killing the plug in that hole. Most guys that have them seem to blip the throttle enough to keep them clean. Overall they are the power kings.

My favorite is a little smaller plenum with a "proper size" butterfly in front of each pair of cylinder's . Much like the E-Tek fronts Monty does with some of his motors.
Open plenum lets it idle like a kitten and with "line of sight" positioning , provides top end power that comes within less than a handful of horses on top ..

[Liberator*21]

Gary,you pose a good question,I think I realized the answer a few years back...coming from carburetors as anyone over 50 did,you understand the one cylinder to one venturi motor design.That one lonely carb throat has to satisfy the needs of its perspective cylinder...only.
Our v6 60 degree motors begin drawing air at #1,then 30 degrees later #2 begins drawing,the two draw together for 60 degrees,then #1drops out and#2 draws the last 30 degrees...then #1 and #2 cylinders are done drawing air and #3 and#4 follow.This 60 degree overlap between #1 and #2 cylinders has the venturi at full flow for both cylinders,but only during the 60 degree overlap,and by cross drilling the intake either(both) cylinders can draw from both carbs at all times,but importantly,during the pre and post 60 degree overlap period when #1 and later #2 benefit from one cylinder drawing from 2 carbs.It gets better...
Now comes the Laser fuel injection airbox,with 4 butterflys.For our airflow purposes,think of them as 4 carb venturies feeding the above 2 cylinders at once,which they are!Lots of air,way more than 1 venturi to one cylinder could provide,unless it was a huge venturi.
The point is,once all cylinders are pulling from a single airbox,the intake on that airbox only needs to be large enough to feed 2 cylinders at any given time.
As to the Downdraft box...in my opinion the design is certainly better,from an airflow potential and production complexity point of view...and I would guess Mercury went with it over the Laser type due to production costs,it is certainly simpler.
Not having done any dyno comparisons I can't say,but my guess is the downdraft probably is'nt a performance enhancer on stock or even moderately modded motors,more flow potential is only potential if it's not needed.The stock downdraft 200's don't seem any better than their laser counterparts in my experiance...we've run both,Chris

Thanks Chris, excellent explanation......

[Liberator*21]

Man I'd love to get my hands on a 225x....I would say its worth it to swap it over depending on what you are bolting it on. It seems, Like Chris said, that they wend to it for production cost reasons...making all their motors run the same intake and throttle linkage. What are you planning on doing inside the 2.5 200? You leaving it stock or making it a beast? ^ Chris said it best, but even if its not a performance gain I think it would be cool to do.

Not really doing anything "MAJOR" to the motor, I did do some port cleanup while it was apart and I was looking for a crank. But other than that, nothing more than cleaned and calibrated the injectors, went to manual fuel pressure reg., shaved heads to 34cc's.

[Liberator*21]

The 4 holes in the Laser add up to more blade area than the single butterfly of a down draft intake. It's downside is that the air has to come in from the back side , make a U-turn .. find it's way past wires , a fuel rail and centrally mounted injectors. those kits that add air from the front tend to confuse the plenum area with colliding columns and still has to deal with the internal mess inside.

The downdraft plenum is a big "still airbox" as it holds holds the same in not more cubic inches of displacement than the engine it provides air for as well as moving the injectors to the outside making for a straight shot from box to reed cage.
The throttle body only needs to be able to replace the volume of air drawn by a single intake signal. I don't think you could rev the motor high enough to run the plenum dry let alone find the limits of the throttle body.

Slides run ratty down low , not all of the opening's seal the same , so some holes run lean while others cut of the air and start killing the plug in that hole. Most guys that have them seem to blip the throttle enough to keep them clean. Overall they are the power kings.

My favorite is a little smaller plenum with a "proper size" butterfly in front of each pair of cylinder's . Much like the E-Tek fronts Monty does with some of his motors.
Open plenum lets it idle like a kitten and with "line of sight" positioning , provides top end power that comes within less than a handful of horses on top ..

QUOTE:
E-Tek fronts Monty does with some of his motors.
Very similar to direct port injection, it really doesn't get any better than that unless there is an air boost assist in the mix somewhere.

Thanks Chaz